Electrical machines, such as motors and generators, have electrical conductors, an electrical insulation, and a laminated stator core. The reliability of the insulating system here is critically responsible for their operational reliability. The insulating system has the function of insulating electrical conductors (wires, coils, bars) durably from one another and from the laminated stator core or the environment. Within high-voltage insulation, distinctions are made between insulation between partial conductors (partial conductor insulation), between the conductors or windings (conductor or winding insulation), and between conductor and ground potential in the slot and winding-head region (main insulation). The thickness of the main insulation is adapted both to the nominal voltage of the machine and to the operational and fabrication conditions. The competitiveness of future plants for energy production, their distribution and utilization, is critically dependent on the materials employed and technologies applied for the insulation.
The fundamental problem with insulators loaded electrically in this way lies in the so-called partial discharge-induced erosion. Under mechanical or thermal loading in the operation of the machine, cavities may form at the interfaces between the insulation and the conductor or between insulation and the laminated stator core, and sparks may form in these cavities as a result of electrical partial discharges. As a result of the sparks, “treeing” channels may be formed in the insulation. The treeing channels that form may lead ultimately to the electrical breakdown of the insulator. Against this background it is state of the art, for the durable insulation of the voltage-carrying conductors of the stators in rotating machines (motors, generators, turbogenerators, water power generators, wind power generators), to employ mica-based insulation systems.
High-voltage and medium-voltage motors and generators currently employ laminar mica insulation. In these systems, the form-wound coils produced from the insulated partial conductors are enwound with mica tapes and impregnated with synthetic resin primarily in a vacuum pressure impregnation (VPI) procedure. Here, mica is used in the form of mica paper, and in the course of the impregnation, the cavities located in the mica paper between the individual particles become filled with resin. The assembly formed of impregnating resin and mica carrier material provides the mechanical strength of the insulation. The electrical strength comes about from the multiplicity of solid-solid interfaces in the mica used. The resulting layering of organic and inorganic materials forms microscopic interfaces whose resistance to partial discharges and thermal stresses is determined by the properties of the mica platelets. As a result of the complicated VPI procedure, even very small cavities in the insulation must be filled fully with resin, in order to minimize the number of internal gas-solid interfaces.
For the additional improvement of the resistance, the use of nanoparticulate fillers is described. It is known from the literature (and through experience when using mica) that inorganic particles, in contrast to the polymeric insulating material, become damaged or destroyed only to a greatly restricted extent, or not at all, on exposure to partial discharge. The resultant erosion inhibition effect is dependent on factors including the particle diameter and the particle surface which generates from it. It is found here that the greater the specific surface area of the particles, the greater the erosion inhibition effect on the particles. Inorganic nanoparticles have very high specific surface areas, at 50 m2/g or more.
Employed for this purpose are essentially the following technologies:                vacuum pressure impregnation technology (VPI process)        resin rich technology        
The principal difference between the two technologies is the construction and the production of the actual insulating system of the coils. Whereas the VPI system is complete only after impregnation and after curing of the winding in a forced-air oven, the leg of the resin rich coil, cured separately under temperature and pressure, constitutes a functioning and testable insulation system even before installation into the stator.
The VPI process operates with porous tapes, forming a solid and continuous insulating system under vacuum with subsequent exposure of the impregnating vessel to overpressure after curing in the forced-air oven.
In contrast to this, the manufacture of resin rich coils is more complex, since each coil leg or winding bar has to be manufactured individually in specific baking presses, leading to a specific increase in the costs of the individual coil. In this context, mica tapes are employed that are impregnated with a polymer insulating substance which is present at what is called a B-stage. This means that the polymer, usually aromatic epoxy resins (BADGE, BFDGE, epoxidized phenol novolaks, epoxidized cresol novolaks, and anhydrides or amines as hardeners), is partially crosslinked and is thus in a tack-free state, but on further heating is able to melt again and be ultimately cured, so as to be brought into the final shape. Since the resin is introduced in an excess, it is able, during the final pressing operation, to flow into all cavities and voids, in order to attain the corresponding quality of insulation. Excess resin is pressed out of the initial charge by the pressing operation. From the literature it is known that the use of nanoparticulate fillers in polymeric insulating materials leads to significant improvements in the insulation in respect of the electrical longevity.
EP 1366112 B1 describes a system which describes the production and properties of a nanoparticulate polymer. Described therein is a polymer with nanoparticulate filler based on silicon dioxide, with a distribution curve having a full width at half maximum of not more than 1.5 dmax.
A disadvantage of the solution proposed there is that the insulation proposed therein is not yet at an optimum in terms of the formation of a passivation coat. A passivation coat is formed by application of an insulating material when a polymer filled with nanoparticles is exposed to partial discharges. Under partial discharge load, the polymeric matrix degrades and releases the filler, in other words, for example, the nanoparticles, which then form a firmly adhering coat on the surface and hence passivate the elements coated with the insulation. In the case of the aforementioned EP 1366112 B1, the passivation coat takes a long time to form, and the agglomeration is incomplete.